Cutting tool assemblies typically include a cutter such as a drill, a tap, a milling cutter etc., a tool holder for securing the cutter, a locking device for locking the tool holder which provides the means by which the locking device grips the tool holder and thereby secures the same. The cutting tool assembly may also include a housing for enclosing the components of the cutting tool assembly and securing them in the desired position. The housing may be in the form of a tool block, a machine spindle, etc.
The locking device has a slot or cavity for receiving the retention knob. The slot has a shape which allows the retention knob to be securely grasped and retained in the desired position to insure precise alignment of the tool holder.
The retention knob commonly accepted in the industry has a cylindrical shaft with a continuous recess circumscribing the outer surface thereof. The recess is comprised of a circular base bound by opposed walls extending upward from the base toward the outer surface of the shaft.
The size of the retention knob including the diameter of the recess is dependent on the size of the shank according to standards in the industry. Relatively large size shanks have correspondingly large size retention knobs wherein the diameter of the recess is generally sufficient to withstand fracture even under severe locking and application forces generated during machining operations. However, smaller shanks (e.g. #20 ANSI standard) having a correspondingly smaller cross-sectional area are more susceptible to fracture along the diameter of the recess. This is because the recess of the retention knob has the smallest cross-sectional area of the entire cutting tool assembly. When the recess fractures, the tool holder separates from the locking device which can result in significant downtime of the machine as well as damage to the tool holder, the cutter, and other components of the cutting tool assembly as well as the workpiece. In addition, retention knobs are often manufactured integral with the tool holder. If the retention knob fails, the tool holder must be discarded which adds significantly to the overall cost of machine tools.
It is therefore an object of the invention to provide a retention knob which has increased strength and is significantly more resistant to fracture than known retention knobs.
It is another object of the invention to provide a retention knob having a greater cross-sectional area for a given sized tool holder than known retention knobs.
It is still another object of the invention to provide a cutting tool assembly employing a retention knob which is less susceptible to fracture, especially those employing relatively small tool holders.